What is the definition or description of: carotid artery dissection?

A tear in the vessel: Arteries are made up of several layers much like an onion. Occaisionally a small tear in the most inner layer can occur. The blood flows through the artery under high pressure. This jet of blood can separate the layers causing the vessel to obstruct (close off). In the carotid artery this could cause a stroke.

Tear or disruption : Tear or disruption between the three layers of the artery wall which start at one location and can extend , enlarge, forms clots with embolization or thrombose and occlude the artery, in rare cases it can rupture but not typically in the carotids . The major problem in carotid dissection is stoke.